Multi-point communications systems having a primary site that is coupled for communication with a plurality of secondary sites are known. One such communication system type is a cable telephony system. Cable telephony systems transmit and receive telephone call communications over the same cable transmission media as used to receive cable television signals and other cable services.
One cable telephony system currently deployed and in commercial use is the Cablespan 2300 system available from Tellabs, Inc. The Cablespan 2300 system uses a head end unit that includes a primary transmitter and primary remote disposed at a primary site. The head end unit transmits and receives telephony data to and from a plurality of remote service units that are located at respective secondary sites. This communication scheme uses TDM QPSK modulation for the data communications and can accommodate approximately thirty phone calls within the 1.9 MHz bandwidth typically allocated for such communications.
As the number of cable telephony subscribers increases over time, the increased use will strain the limited bandwidth allocated to the cable telephony system. Generally stated, there are two potential solutions to this bandwidth allocation problem that may be used separately or in conjunction with one another. First, the bandwidth allocated to cable telephony communications may be increased. Second, the available bandwidth may be used more efficiently. It is often impractical to increase the bandwidth allocated to the cable telephony system given the competition between services for the total bandwidth assigned to the cable service provider. Therefore, it is preferable to use the allocated bandwidth in a more efficient manner. One way in which the allocated bandwidth may be used more efficiently is to use a modulation scheme that is capable of transmitting more information within a given bandwidth than the TDM QPSK modulation scheme presently employed.
The present inventors have recognized that OFDM/DMT modulation schemes may provide such an increase in transmitted information for a given bandwidth. U.S. Pat. No. 5,539,777, issued Jul. 23, 1996, purports to disclose a DMT modulation scheme for use in a communications system. The system principally focuses on applications in which a single secondary site includes a plurality of differing remote and transmitter devices. The transmitters and remotes used at the secondary site of the system described therein, however, are quite complex and require a substantial amount of processing power. As such, the system disclosed in the '777 patent does not readily or economically lend itself to multi-point communications systems in which there are a large number of secondary sites each having at least one remote.
Another concern with multipoint communications systems relates to the establishment of communications from a particular remote service unit to the head end unit receiver (up-stream). Prior to communication, the remote unit is not registered or synchronized over any upstream channel. In the past, if an unsynchronized, unregistered remote unit attempted to transmit in the upstream direction while other remote units were transmitting in the upstream direction, the unsynchronized remote unit would disrupt in-progress data transmissions with the other remote units.
Past systems have addressed this problem in the manner described in a paper entitled "Synchronized DMT for Multipoint-to-Point Communications on HFC Networks", by Jacobsen et al., 1995, published at the November, 1995 IEE Global Telecommunications Conference in Singaport. The Jacobsen et al. paper explains the conventional approach to synchronize discrete multi-tone data (DMT) for multipoint to point communications as follows. The network defines a silent interval in every upstream channel from the remote units. The silent interval has a predetermined length and is observed periodically by all synchronized remote units presently registered with the network. The unsynchronized remote units seeking to register with the network were only permitted to transmit upstream during the silent interval, at which time they requested an upstream channel assignment (e.g., they transmitted an installation signal). Once the head end unit received the request, it and the remote unit performed synchronization operations. The head end unit also registered the remote unit at that time. The duration of the silent interval was required to exceed at least the maximum round-trip signal delay over all remote units plus the time required to actually transmit the installation information. However, the foregoing conventional approach to upstream synchronization has drawbacks. For instance, the conventional approach requires that all remote units cease transmitting upstream during the silent interval, thereby substantially reducing the available upstream transmission capacity.
Therefore, a need remains for an improved upstream registration/synchronization method and apparatus.